feat(sync): envr can now detect if directories have moved.

app/features.go

@@ -1,9 +1,20 @@
 package app
 
 import (
+	"fmt"
 	"os/exec"
 )
 
+type MissingFeatureError struct {
+	feature AvailableFeatures
+}
+
+func (m *MissingFeatureError) Error() string {
+	return fmt.Sprintf("Missing \"%s\" feature", m.feature)
+}
+
+// TODO: Features should really be renamed to Binaries
+
 // Represents which binaries are present in $PATH.
 // Used to fail safely when required features are unavailable
 type AvailableFeatures int
@@ -30,3 +41,20 @@ func checkFeatures() (feats AvailableFeatures) {
 
 	return feats
 }
+
+// Returns a MissingFeature error if the given features aren't present.
+func (a AvailableFeatures) validateFeatures(features ...AvailableFeatures) error {
+	var missing AvailableFeatures
+
+	for _, feat := range features {
+		if a&feat == 0 {
+			missing |= feat
+		}
+	}
+
+	if missing == 0 {
+		return nil
+	} else {
+		return &MissingFeatureError{missing}
+	}
+}